A large variety of optical deflectors having a resonance-oscillated mirror have been proposed. The resonance type optical deflector has the following features, as compared with a conventional optical-scanning optical system using a rotary polygonal mirror such as a polygon mirror: that is, the size of the device can be made very small and the power consumption can be kept low. Particularly, an optical deflector comprising Si monocrystal, which is produced through the semiconductor process, has theoretically no metal fatigue and thus the durability thereof is very good.
On the other hand, in the resonance type deflector, since the deflection angle (displacement angle) of the mirror theoretically changes sinusoidally, the angular speed of deflected light is not constant. The following techniques have been proposed to correct this characteristic (see U.S. Pat. Nos. 4,859,846 , 5,047,630 and U.S. Patent Application Publication No. 2006/0,152,785).
In U.S. Pat. Nos. 4,859,846 and 5,047,630, a resonance type deflector having an oscillation mode based on a fundamental frequency and a frequency three-fold the fundamental frequency, is used to achieve the driving in which the deflection angle of the mirror changes like a chopping wave.
FIG. 8 shows a micromirror which realizes such chopping-wave drive. In FIG. 8, an optical deflector 12 is comprised of oscillators 14 and 16, elastic supporting members 18 and 20, driving members 23 and 50, detecting elements 15 and 32, and a control circuit 30. This micromirror has a fundamental resonance frequency and a resonance frequency approximately three-fold the fundamental frequency, and it is driven at a combined frequency of the fundamental frequency and the three-fold frequency. Based on this, the oscillator 14 having a mirror surface is driven by a chopping-wave drive, such that optical deflection with a smaller angular speed change in the deflection angle is accomplished as compared with the sinusoidal drive.
During the driving, the oscillation of the oscillator 14 is detected by the detecting elements 15 and 32, and a driving signal necessary for the chopping wave is generated by the control circuit 30. The driving signal is inputted to the driving members 23 and 50, by which the micromirror is driven. As described above, since the angular speed of the scanning deflection has an approximately-constant angular-speed region which is wide as compared with a case where the displacement angle is based on a sinusoidal wave, the available region relative to the whole area of scanning deflection can be widened.
Furthermore, the drive is performed in accordance with the fundamental frequency and a frequency three-fold the fundamental frequency or, alternatively, a driving frequency based on a three-fold frequency and a one-third frequency of that.
In an oscillation system comprising a plurality of oscillating movable elements and a plurality of elastic supporting members, a drive close to a saw-tooth wave drive or a triangular-waveform drive can be realized by setting the resonance frequency ratio of plural natural oscillation modes for the driving to an integral-number ratio. However, due to a manufacturing error or an environmental change, it is difficult to set the resonance frequencies of the natural oscillation modes of the oscillator device exactly at the integral-number ratio.
For example, it is now assumed that the resonance frequency f1 of a first natural oscillation mode and the resonance frequency f2 of a second natural oscillation mode are not in a precisely two-fold relationship but rather they are in an approximately two-fold relationship. Here, the driving will be carried out while a first driving frequency Df1 is made equal to the resonance frequency f1 of the first natural oscillation mode, and a second driving frequency Df2 is set to a frequency n-fold the first driving frequency Df1 (n is an integer). Then, the second driving frequency Df2 would be largely deviated from the resonance frequency f2 of the second natural oscillation mode. As a result of this, degradation of the driving efficiency and the like occurs, and it becomes very difficult to obtain the driving of the movable element exactly in accordance with a desired scanning waveform. This problem occurs when the relationship of the resonance frequencies of plural natural oscillation modes does not match the design value (it is deviated from the designed integral-number ratio). For example, a similar problem may occur even in a case where the ratio is approximately an integral-number ratio other than approximately two-fold (e.g., approximately three-fold). Here, the word “approximately” is referred to so as to exclude a case where the ratio is exactly an n-fold (n is an integer).